The present invention relates to the use of bromo or chloro derivatives of stilbene to enhance the oxygen index of alkenyl aromatic resins, particularly expandable alkenyl aromatic resins.
It is known in the art that the relative flammability of various plastic materials may be reduced by incorporating into the plastic one or more additives collectively referred to as flame retardants prior to heat processing of the plastic into shaped articles. This incorporation may be accomplished by physical blending such as using an extruder, or in some cases by polymerizing the resin forming monomer or monomer mixture in the presence of a dispersion or solution of the flame retardant.
Among the various flame retardants known in the art are certain compounds containing phosphorus, filler materials such as glass fiber, talc or alkali silicates, inorganic salts such as the borates or ammonium salts, and organic compounds containing halogen. A great many of the preferred organic compounds contain phosphorus, bromine or chlorine, alone or in combination, with the organic moiety being aliphatic, cycloaliphatic or aromatic. Common flame retardants of this latter category include such materials as tetrabromobutane, hexabromoethane, hexabromocyclohexane, tetrabromovinylcyclohexane, decabromodiphenyl ether, hexabromobenzene, decabromobiphenyl, dibromoneopentyl glycol, tetrabromobisphenol-A, tris-(2,3-dibromopropyl) phosphate and other materials. Chlorinated or mixed bromo-chloro homologues of many of these materials have also been successfully employed.
It is also known in the art that various additives incorporated into plastic material enhance the action of many flame retardants, although the precise mechanism of this enhancement is not known. Common additives include the oxides and sulfides of antimony, arsenic and bismuth, the most popular of this class being antimony oxide. Other known additives are categorized as free radical initiators and include thermally unstable organic compounds such as peroxides, nitroso and azo compounds, and structurally hindered compounds.
Within this rather general context, the use of nucleus brominated or mixed bromo-chloro halogenated derivatives of di-aromatic compounds as a flame retardant is disclosed in U.S. Pat. No. 3,141,860. According to the patent disclosure a mixture of the halogen compound, which must contain at least 3 bromine atoms attached to an aromatic nucleus and also contain at least 50% by weight bromine, and antimony trioxide imparts suitable flame retardancy to polyolefin plastics. U.S. Pat. No. 3,331,797 discloses flame resistant polyolefin compositions comprising a mixed system of antimony trioxide, a chlorinated paraffin and a halogenated aromatic compound which includes a polybromo diphenyl ethers as well as di-aromatic compounds taught in U.S. Pat. No. 3,141,860, discussed above. While these halogenated additives are effective for the purposes taught in the patents, they are relatively difficult to synthesize and are relatively expensive due in large measure to the high bromine content. Also, because relatively large quantities (5 to 20% by weight) of the halogenated additive must be used to achieve desired results, the additives can adversely affect the physical properties of the polymers.
Whereas these and other prior art flame retardant compositions are generally effective, it is most desirable to provide new flame retardant compositions containing compounds which offer relatively greater efficiency and are readily synthesized and readily available.